1. Field of the Invention
The present invention relates to nano-structures and devices using the same, and a process for preparing the nano-structures. Particularly, the present invention relates to nano-structures having pores, which is believed to be widely used as, for example, electronic and optical devices, functional materials for micro-devices, structural materials, etc., devices using the nano-structures, and a process for preparing the nano-structures.
2. Description of the Related Art
Some thin films, wires and dots of metals or semiconductors, which have sizes smaller than a certain length, exhibit specific electrical, optical and chemical properties due to enclosure of electron movement. From this viewpoint, materials (referred to as xe2x80x9cnano-structuresxe2x80x9d hereinafter) having a fine structure of several 100 nm or less have increasingly attracted attention as functional materials.
An example of processes for preparing such nano-structures comprises preparing a nano-structure directly by a semiconductor processing technique such as a patterning technique such as photolithography, electron beam exposure, X-ray exposure, or the like.
Besides this preparing process, an attempt is made to realize a novel nano-structure comprising a regular structure naturally formed, i.e., a structure formed in a self-ordering manner, as a base. This process can possibly produce a specific fine structure superior to structures produced by conventional processes depending upon the fine structure used as the base, and thus many studies have been conducted.
An example of such a self-ordering process is anodic oxidation which can easily produce a nano-structure having pores in nano-size with high controllability. For example, anodic porous alumina formed by anodically oxidizing aluminum or an alloy thereof in an acidic bath is known.
Anodic oxidation of an Al plate in an acidic electrolyte forms a porous oxide film (anodic porous alumina) (refer to, for example, R. C. Furneaux, W. R. Rigby and A. P. Davids on NATURE Vol. 337, P147 (1989)). The porous oxide film is characterized by having a specific geometric structure in which very fine cylindrical pores (nano-holes) having a diameter of several nm to several hundreds nm are arranged in parallel at intervals (cell size) of several nm to several hundreds nm. The cylindrical pores have a high aspect ratio, and are excellent in uniformity of the sectional diameter. The diameter and interval of the pores can be controlled to some extent by controlling the current and voltage in anodic oxidation, and the thickness of the oxide film and the depth of the pores can be controlled to some extent by controlling the anodic oxidation time.
In order to improve the perpendicularity, linearity, and independence of the pores of the anodic porous alumina, a two-step anodic oxidation process has been proposed, in which after a porous oxide film formed by anodic oxidation is removed, anodic oxidation is again performed to form anodic porous alumina (ordered alumina nanohole) having pores having good perpendicularity, linearity and independence (Jpn. Journal of Applied Physics, Vol. 35, Part 2, No. 1B, pp. L126-L129, Jan. 15, 1996). This process utilizes the property that surface concaves of an aluminum plate formed by removing the anodic oxide film formed by first anodic oxidation serve as the starting points of pore formation in second anodic oxidation.
Besides these processes, the process of forming pore starting points by using press pattering has also be proposed, in which a substrate having a surface comprising a plurality of convexes is pressed on the surface of an aluminum plate to form concaves as pore starting points, and then anodic oxidation is performed to form a porous oxide film having pores exhibiting good shapes, interval and pattern controllability (Japanese Patent Laid-Open No. 10-121292).
In consideration of the specific geometric structure of the anodic porous alumina, various applications are attempted. Although this is explained in detail by Masuda, examples of application are described below. Examples of applications include applications to films using the anodically oxidized film having abrasion resistance and insulation resistance, applications to filters using separated films, etc. Furthermore, various other applications to coloring, magnetic recording media, EL light emitting devices, electro-chromic devices, optical devices, gas sensors, etc., are attempted by using the technique of filling nano-holes with a metal, a semiconductor, or the like, and the technique of replicating the nano-hole structures. Furthermore, applications to various fields of quantum fine wires, quantum effect devices such as a MIM device, a molecular sensor using nano-holes as chemical reaction fields, etc. are expected (Masuda, Solid State Physics, 31, 493 (1996)).
Since the above-mentioned direct preparation of nano-structures by the semiconductor processing techniques has the problems of low yield and high equipment cost, a simple preparation process having high reproducibility is demanded.
From this viewpoint, the self-ordering process, particularly the anodic oxidation process, is preferred because it can easily prepare nano-structures with high controllability, and prepare nano-structures in a large area. Particularly, the structure of anodic porous alumina formed by two-step anodic oxidation or press patterning, in which pores are regularly arranged, are preferred from the viewpoint of structural uniformity of perpendicularity, linearity, and arrangement of the pores.
In the process of studying applications of nano-structures to devices, the inventors confirmed that an arrangement of two kinds of pores having different diameters at controlled positions in a nano-structure permits expansion of the range of applications of nano-structures to devices. For example, it is expected that a material having a structure in which the dielectric constant (refractive index) periodically changes in a cycle of length near the wavelength of light produces photonic crystals, thereby permitting a high degree of light control. More specifically, a photonic band gap in which the presence of light is inhibited in a predetermined wavelength range is formed, or light is localized in a predetermined wavelength range to enable applications of non-structures to a light guide, a light emitting device, etc. One of the two kinds of pores having different diameters can be possibly used as photonic band gap regions, or regions where light is localized. In addition, in filling pores having different diameters with a magnetic material, the strength of a magnetic field required for reversing the magnetization direction possibly changes with changes in diameter of the pores. This can be possibly applied to, for example, formation of tracking tracks on a recording medium.
An example of conventional known methods of controlling the diameters of the pores of anodic porous alumina is to immerse alumina in an acidic solution (pore widening). However, this method basically controls the pores to the same diameter, and cannot control independently the diameters of the pores.
As a result of repetition of various studies in consideration of the above-described technical background, the inventors found a method for forming a nano-structure in which at least two kinds of pores having different diameters are respectively arranged at controlled positions, leading to the achievement of the present invention.
Accordingly, an object of the present invention is to provide a nano-structure having a construction for widening the range of applications to various devices, and a light emitting device, an optical device and a magnetic device using the same.
Another object of the present invention is to provide a process for preparing a nano-structure having a novel construction for widening the range of application to devices having a novel structure.
In accordance with a first aspect of the present invention, there is provided a nano-structure comprising an anodically oxidized layer, wherein the anodically oxidized layer comprises a plurality of kinds of pores.
In accordance with another aspect of the present invention, there is provided a nano-structure comprising an anodically oxidized layer containing a first pore and a second pore, wherein the diameter of the first pore is different from that of the second pore, and the first and second pores are respectively provided at controlled positions in the layer.
In accordance with still another aspect of the present invention, there is provided a light emitting device comprising a nano-structure comprising an anodically oxidized layer having a plurality of kinds of pores, wherein the pores are filled with a material having a luminescent function.
In accordance with a further aspect of the present invention, there is provided a light emitting device comprising a nano-structure comprising an anodically oxidized layer containing a first pore and a second pore having different diameters, wherein the first and second pores are respectively provided at controlled positions in the anodically oxidized layer, and at least one of the first and second pores is filled with a material having a luminescent function.
In accordance with a further aspect of the present invention, there is provided a light emitting device comprising a nano-structure comprising an anodically oxidized layer having a plurality of kinds of pores, wherein the pores are filled with a material having a refractive index different from that of the anodically oxidized layer.
In accordance with a further aspect of the present invention, there is provided a light emitting device comprising a nano-structure comprising an anodically oxidized layer containing a first pore and a second pore having different diameters, wherein the first and second pores are respectively provided at controlled positions in the anodically oxidized layer, and at least one of the first and second pores is filled with a material having a refractive index different from that of the anodically oxidized layer.
In accordance with a further aspect of the present invention, there is provided a magnetic device comprising a nano-structure comprising an anodically oxidized layer having a plurality of kinds of pores, wherein the pores are filled with a magnetic material.
In accordance with a further aspect of the present invention, there is provided a light emitting device comprising a nano-structure comprising an anodically oxidized layer containing a first pore and a second pore having different diameters, wherein the first and second pores are respectively provided at controlled positions in the anodically oxidized layer, and at least one of the first and second pores is filled with a magnetic material.
In accordance with a further aspect of the present invention, there is provided a process for preparing a nano-structure comprising an anodically oxidized layer having a plurality of kinds of pores, the process comprising the steps of preparing a film containing aluminum and having a plurality of kinds of starting points for the respective pores on a surface thereof, and anodically oxidizing the film containing aluminum, wherein the plurality of kinds of staring points are different in at least one of shape and composition.
In accordance with a further aspect of the present invention, there is provided a process for preparing a nano-structure comprising an anodically oxidized layer having first and second pores having different diameters, the process comprising the steps of preparing a film containing aluminum and having first and second starting points for the respective pores on the surface thereof, and anodically oxidizing the surface, wherein the first and second starting points are different in at least one of shape and composition.
The nano-structure having the above construction is formed by forming pore starting points at desired positions in a workpiece, and then anodically oxidizing the workpiece. In forming the pore starting points, the shape or composition of each of the pore starting points is controlled to independently control the diameters of the respective pores of anodic porous alumina. This method can realize a porous material having pores which have desired diameters and are regularly arranged at desired positions.
In the nano-structure of the present invention, the pores may be filled with a functional material such as a metal, a semiconductor, or the like to cause the possibility of application to new electronic devices.
The nano-structure of the present invention can also be used as a mold or mask to form a new nano-structure. For example, a porous material having through pores, which is obtained by removing portions of the nano-structure of the present invention other than the porous portion, can be used as a mask for deposing a functional material such as a metal, a semiconductor, or the like, or provided as an etching mask on another substrate, to form a nano-structure for quantum dots, or the like.
The nano-structure of the present invention can be used for various applications such as a quantum wire, a MIM element, a molecular sensor, coloring, a magnetic recording medium, an EL light emitting device, an electro-chromic device, an optical device such as a photonic crystal, an electron emitting device, a solar cell, a gas sensor, an abrasion resistant-insulating resistant film, a filter, etc. The nano-structure has the function to widen the range of application thereof.
Particularly, a material having a structure in which the dielectric constant periodically changes in a cycle of a length near the wavelength of light forms photonic crystals, and has the possibility of enabling a high degree of light control. More effectively, a photonic band gap appears, in which the presence of light is inhibited in a predetermined wavelength range (Photonic Crtstals, J. D. Joannopoulos, R. D. Meade, and J. N. Winn, Princeton University Press). Anodic porous alumina having a regular arrangement of pores can be used as a photonic crystal by utilizing the periodic structure thereof. In the present invention, the technique of independently controlling the diameters of pores of anodic porous alumina having regularly arranged pores permits control of the structure of a photonic crystal, control of the structure of a photonic band, and the formation of a waveguide or defects. In the photonic crystal, a localized state of light can be obtained by introducing defects, and thus a localized state of light can be obtained by locally changing the diameters of some of the pores of anodic porous alumina. This permits further application to optical recording media, and the like.
With a photonic crystal comprising a luminescent material arranged therein, a photonic band is appropriately designed according to the emission wavelength to permit control of spontaneous emission, and an improvement in performance of a light emitting device can thus be expected. Namely, the pores of the above-described anodic porous alumina are filled with a luminescent material to make it possible to expect the realization of a light emitting device with a low threshold value, a light emitting device with a narrow emission spectral width, a laser with a low threshold value, etc.
Furthermore, the pores of the anodic porous alumina are filled with a magnetic material to obtain magnetic nano-wires, and the pores of the anodic porous alumina, which have different diameters, are filled with a magnetic material to form an arrangement of magnetic fine wires having different diameters. Since the size of a magnetic fine wire affects the threshold of magnetization reversal, and domain control, magnetic resistance, etc. in a fine wire, the control of these properties enables application to magnetic devices such as a magnetic sensor, a magnetoresistive element, a magnetic recording medium, and the like.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.